Method of making calcined minerals with reduced sulfur content

ABSTRACT

A method for reducing the sulfur impurity content of basic flux material such as calcium or magnesium oxides in which the kiln atmosphere is changed from oxidizing to reducing during the latter part of the calcining operation.

United States Patent I l l Inventor Appl. No.

Filed Patented Assignee Carl E. Sunnegren Bethlehem, Pa.

Jan. 12, 1970 Jan. 11, 1972 Bethlehem Steel Corporation [56] References Cited UNITED STATES PATENTS 2,945,688 7/1960 Pajenkamp et a1. 263/53 3,425,853 2/1969 Rives 263/53 3,499,636 3/1970 Hall 263/53 X METHOD OF MAKING CALCINED MINERALS WITH REDUCED SULFUR CONTENT 6 Claims, 3 Drawing Figs.

U.S. Cl 263/53 R Int. Cl C04b 1/02 Field of Search 263/53, 53 A; 106/101 STONE ENTRY POI NT l COMBUSTION PRODUCTS FLOW X BURNERS TC THERMOCOUPLES ASP- ATMOSPHERE SAMPLE POINTS Primary Examiner-John .I. Camby Attorney-Joseph .l. O'Keefe ABSTRACT: A method for reducing the sulfur impurity content of basic flux material such as calcium or-magnesium 0xides in which the kiln atmosphere is changed from oxidizing to reducing during the latter part of the calcining operation.

LIME DISCHARGE l l I TC-l l I l l I l E4 l COMBUSTION TC-Z B22 ASP(2) 6 B23 PROBE 1 2 METHOD OF MAKING CALCINED MINERALS WITH In a typical calcining process limestone, or dolomite, is REDUCED SULFUR CONTENT charged onto, or into, the feed end of'the kiln and the oxide is BACKGROUND OFTHE mvsmsm removed either intermittently or continuously from the other end. The reaction for calcium carbonate is:

This invention is directed to a method of producing a cal- 5 a' camcooz o cined basic oxide in which the sulfur, present as an impurity, lp f range lf from L to 23500 R has been substantially reduced Kiln design varies, but in general most kilns fall into the ln basic iron and Steel manufacture, lime (C30), and/or class heading of vertical, rotary or fluid bed,- with wide variadolomite (CaO and MgO) are used as fluxes for slagging, or m W Miler these headlflgs' removal, of impurities such as silica and sulfur. With the in- A popular of recent vmtage the c'rcular travelmg troduction of basic oxygen furnaces, the amount of flux hearth kiln which features a hearth of large diameter that can needed per ton of steel has increased, and at the same time the be operated at various speeds of 35 to 200 Control requirements for the flux purity have become more stringent lability of this kiln is unique since the hearth is divided into Maximum tolerance for sulfur in flux is about 0.05 percent. 1 5 heating Zones through mstmmematlon prisclse tempera The raw materials for basic flux are the naturally occurring tures can be maintained uniformly at these different zones. carbonates of the two minerals calcium and magnesium. The Heat is supplied by multiple burners. The stone charge is fed overall calcininguemoval b )reaction from a preheated chamber onto the hearth in an even bed caco (320K302 while a continuous loop drag conveyor removes all lime at the and 3 2O discharge point as it completes a revolution. In the practice of Mgco3 Mgo+co2 my invention air or the gas supply to the burners which control Ideally, this reaction must be carried out at a sufficiently the hfeat SuPplied F the latter h of the kiln or Portion of high temperature and time length to cause the dissociation of the kiln prior to discharge of lime from the calcining zone, are the carbonate to he oxide carbon dioxide but not high set to cause a reducing flame combustion. The atmosphere in enough, nor long enough to produce lime of reduced activity, that of the hearth ls therefore Starved known as nhardbumed the reduction of the oxygen present, the reactions: Sulfur may be present in the native stone in several forms, a+ 2+ 2 4+ 2 notably as the sulfite, sulfate or iron pyrite. Some may also be caolsofl'fioa CaSQt present as fnememal ]f are no longer as favored as they are in an oxidizing at- Because of the difficulty of sulfur removal, only flux materimosphel'egytztill tnaturally low sulfur content have been utilized in the BRIEF DESCRIPTION OF THE DRAWINGS It is the object of this invention to produce a reactive basic FIG. 1 is a graph of the free energy change AFR of the imoxide with a sulfur impurity content low enough to render it portant sulfur reactions with calcium carbonate in the calcinsuitable for use in the steelmaking process. ing process over a temperature range of 1,400 to 2,500 F.

FIG. 2 shows an average of the percent sulfur content in SUMMARY OF THE INVENTION lime when ox en is su ressed in the kiln atmos here.

y PP P I have discovered that the aforementioned object can be 3 is aischemflfic drawing Ofa'circulal' traveling hearth achieved by suppressing the oxygen present in the kiln at- 40 with the burner P l thermocouPle Sample PolmS TC, mosphere during the latter part of the calcining process. The and atmosphere SamPle Pomts ASP Showncalcining process being defined to include drying the stone if DESCRIPTION OF THE PREFERRED EMBODIMENT necessary as well as heating the carbonate to cause dissociation. When the atmosphere is changed to a reducing condition Referring to FIG. 1, a negative value of the standard free production of calcium sulfate and calcium sulfite from various energy change shows there is a tendency, or driving force, for sulfur compounds present is reduced. In addition the amount a reaction to occur. The less negative the value, the less the of the calcium sulfate and calcium sulfite present in the native tendency. Curves 2 and 4 differ from I and 3 in that when oxore is also reduced. Sulfur, present as the sulfate, sulfite, sulygen is present there is a tendency for calcium sulfate to form. tide and elemental sulfur, are continually converted to gase- Referring to FIG. 3, burners 1 through 11 on the outside ous sulfur compounds which are vented along with the carbon dioxide produced from the calcining step.

When sulfur is the impurity present it is capable of entering into the reaction in the form of SO or S0 and, in the example of calcium, the following reactions may occur:

TABLE A.KILN ATMOSPHERE ANALYSES AT VARIOUS POINT wall and burners 22 through 27 on the inside wall are set to burn with a reducing flame. Burners 12 through 21 function as standard oxidizing burners. The proper fuel/air ratio to the burners is maintained by atmosphere analysis. A typical exam pie is a s fo ll ows:

[Sample Point (See FIGURE 3)] l #6 burner, Probe, #12 burner, #15 burner #19 burner, Near flue, Port #1, percent 0 percent C percent, 0 percent 0 percent 6 I percent 02 percent 02 l C =combustibles. 2 Average. A

It is important to note that S0 is a gas, while the sulfites and the material to very high temperatures. This could result in the production of the undesirable hard-bumed lime.

This sample analysis shows that the reducing flame burners, although covering only 35 percent of the active hearth are sufficient to generate a reducing atmosphere (one that contains an excess of combustible material or fuel) in at least 50 percent of the active hearth area. Analysis at burner 15, the approximate halfway point in the hearth shows combustibles present, while the atmosphere at burner 19 shows oxygen to be present.

Referring again to FIG. 3, the stone charge, which has been crushed and preheated using standard techniques well known have used natural gas and coke oven gas as fuels, coke, coal, oil, their mixtures or other suitable fuels can be used.

lclaim:

1. A method of treating at least one of the minerals of the group onsisting of calcium carbonate, and magnesium car- TABLE B.-KILN TEMPERATURE PATTERN the entrance point, to a little above 2,400 F. at the midpoint of the kiln. The average time for a complete rotation is about 80 min. The lime in these samples contains an average of 0.036 percent sulfur after the treatment, which is an average of 78 percent of the sulfur removed.

The best temperature to use in the reducing zone is a variable within a range, and in general depends on the percent total sulfur to be removed. This means that not only must the sulfur present in the native stone be taken into consideration, but also the sulfur which may be present in the fuel as an impurity. The preferable temperature depends also on the final percent sulfur level to be achieved. In general, the greater the percent sulfur to be removed, or the lower the final percent sulfur level to be achieved, the higher should be either the operating temperature or the length of time the lime is exposed to the operating temperature. It should be noted that while it is not essential to keep the temperature uniformly high during the desulfurizing, it is preferable because of the reversibility of the reaction. The temperature relationship is shown in table C in which dolomite (53.2% CaCO +44.6% MgCO is the sample.

The above sample was run in a tube furnace with an average time of about 90 minutes and using a gas atmosphere approximating kiln conditions, but with no sulfur dioxide in the fuel gases. When a sample of limestone was similarly treated in a kiln, according to my invention, using natural gas as a fuel for the burners, the sulfur percent in the lime was 0.06 at about l,600 F. and dropped to 0.025 when the temperature rose to about 1,800 F.

The effect of time and temperature on the percent sulfur present in the finished sample is illustrated below. The samples are limestone which were heated in a reducing atmosphere in a tube furnace.

It is obvious that there are many variations of this process which are within the scope of my invention. The time and the temperature may vary as well as the composition or particle size of the stone to be treated. Neither is it necessary for this process to be continuous with the calcining operation and it is not limited to any type of kiln or any hearth or refractory composition as long as the atmosphere is controllable. While I Lime analysis, Thermocouple readings, F. percent Percent N o. of lime Sulfur sulfur samples in stone TC 1 TC 2 TC 3 TC 4 TC 5 TO 6 S 00 removed 12 0. 074 1, 736 2, 282 2, 40s 2, 241 1, 895 1, 420 0. 03s 5. 14 69. 7 1 0. 110 1, 784 2, 298 2, 425 2, 258 1, 906 1, 399 0. 036 3. 27 82. 3 20 0. 098 1, 812 2, 293 2, 416 2, 233 1, 889 1, 404 0. 034 1. 55 81. 0 18 0. 080 1, 782 2, 290 2, 397 2, 274 1, 923 1, 421 0. 035 4. 60 76. 7 10 0. 087 1, 864 2, 382 2, 462 2, 320 1, 961 1, 445 0. 029 2. 29 81. 8 Total 77 Average 0. 1, 795 2, 309 2, 421 2, 265 1, 914 1, 417 0. 036 3. 28 78 Referring to the last line of table B, a total of 77 samples B with an average of 0.090 percent sulfur in the stone were TA LE D treated. The kiln temperatures range from about 1,400 F. at 20 Sample No. 1, Initial sulfur of 0.08%

bonate and mixtures thereof, comprising the steps of:

a. calcining said minerals, and

b. desulfurizing said minerals by converting and maintaining nongaseous sulfur and sulfur-containing compounds as gaseous sulfur compounds in a heated furnace wherein the atmosphere in said furnace has an oxygen to fuelratio less than the stoichiometric amount.

2. The method as claimed in claim I wherein the temperature in the furnace in step (b) is from about 1,700 to about 2,500 F. and the time is at least 5 minutes.

3. A method of treating at least one of the minerals of the group consisting of calcium carbonate and magnesium carbonate, and mixtures thereof, in a furnace comprising the steps of:

a. passing said minerals through a first heated zone wherein the atmosphere in said first zone has an oxygen to fuel ratio greater than the stoichiometric ratio, wherein said minerals are calcined, and

b. passing said minerals through a second heated zone wherein the atmosphere in said second zone has an oxygen to fuel ratio less than the stoichiometric ratio and wherein said minerals are desulfurized by converting and maintaining sulfur and sulfur-containing nongaseous compounds as gaseous sulfur compounds.

4. The method as claimed in claim 3 wherein the temperature in the furnace in step (b) is from about l,700 to about 2,500 F. and the time is at least 5 minutes.

5. A method of treating at least one of the minerals of the group consisting of calcium carbonate and magnesium carbonate and mixtures thereof, comprising the steps of:

a. calcining said minerals in a furnace,

b. discharging said minerals from said furnace,

c. charging said minerals in a furnace,

d. converting and maintaining nongaseous sulfur and sulfur compounds as gaseoussulfur compoundsby heating said minerals in said furnace in a heated atmosphere having an oxygen to fuel ratio of less than the stoichiometric ratio to desulfurize said minerals.

6. The method as claimed in claim 5 wherein the temperature in the furnace in step (b) is from about l,700 to 2,500 F. 5 and the time is at least 5 minutes. 

2. The method as claimed in claim 1 wherein the temperature in the furnace in step (b) is from about 1,700* to about 2,500* F. and the time is at least 5 minutes.
 3. A method of treating at least one of the minerals of the group consisting of calcium carbonate and magnesium carbonate, and mixtures thereof, in a furnace comprising the steps of: a. passing said minerals through a first heated zone wherein the atmosphere in said first zone has an oxygen to fuel ratio greater than the stoichiometric ratio, wherein said minerals are calcined, and b. passing said minerals through a second heated zone wherein the atmosphere in said second zone has an oxygen to fuel ratio less than the stoichiometric ratio and wherein said minerals are desulfurized by converting and maintaining sulfur and sulfur-containing nongaseous compounds as gaseous sulfur compounds.
 4. The method as claimed in claim 3 wherein the temperature in the furnace in step (b) is from about 1,700* to about 2,500* F. and the time is at least 5 minutes.
 5. A method of treating at least one of the minerals of the group consisting of calcium carbonate and magnesium carbonate and mixtures thereof, comprising the steps of: a. calcining said minerals in a furnace, b. discharging said minerals from said furnace, c. charging said minerals in a furnace, d. converting and maintaining nongaseous sulfur and sulfur compounds as gaseous sulfur compounds by heating said minerals in said furnace in a heated atmosphere having an oxygen to fuel ratio of less than the stoichiometric ratio to desulfurize said minerals.
 6. The method as claimed in claim 5 wherein the temperature in the furnace in step (b) is from about 1,700* to 2,500* F. and the time is at least 5 minutes. 